Advances in the science and engineering of lithium-ion batteries have resulted in lithium-ion batteries becoming the most popular power source for portable electronic devices, and recently, as a power source for electric and hybrid electric vehicles.
Within the lithium-ion cell, the interface between the anode and the electrolyte is of particular importance, and it is a major factor with regard to overall cell performance. In particular, a thin passivation layer, also called SEI (solid electrolyte interface), is typically formed during the first charging process and prevents further reactions of the electrolyte on the anode surface. For cells utilizing carbon anodes, the SEI formation is potential dependant and stepwise, and is ultimately determined by the reactive component of the electrolyte.
Currently, the state-of-the-art electrolytes for lithium-ion battery utilize lithium hexafluorophosphate (LiPF6) as an electrolyte salt and carbonates, such as ethylene carbonate, as solvents. Ethylene carbonate (EC) prevents the formation of SEI at the surface of the negative electrode so that good battery performances can be achieved. However, in many cases the SEI protection achieved from using electrolytes with simple formulations, such as LiPF6 in EC and/or other carbonates, is insufficient where the negative electrode contains carbonaceous. For instance, when cycling under elevated temperature, the capacities of carbon-anode lithium-ion batteries can fade very quickly. Additionally, when using a non-graphite carbon anode, such as natural graphite and hard carbon (an amorphous non-graphitic carbon), the batteries exhibit a higher initial discharge capacity that is quickly lost in subsequent cycles.
The performance of lithium ion batteries containing EC-based electrolytes is limited to low temperature applications. For higher temperature applications, propylene carbonate (PC) has been considered to fully or partially replace EC as PC remains liquid over a wider temperature window (−55 to 240° C.) than EC. LiPF6—PC-based electrolytes, however, are not compatible with graphite electrodes due to exfoliation of the graphite structure by PC intercalation.
However, there is still a need for effective additives and/or co-solvents in the electrolyte of lithium-ion batteries to prevent the formation of SEI films on graphic, or other carbonaceous electrodes, to prevent or reduce further decomposition of solvent molecules over long service lives, and/or to prevent the intercalation of PC into graphite anodes.